1. Field of the Invention
The present invention relates to toner content control of an image forming apparatus such as a laser beam printer.
In an electrophotographic or electrostatic recording unit, a developing unit is used to visualize (develop) an electrostatic latent image formed on a photosensitive plate or drum surface. In a developing unit using powder developer, especially developer consisting of a mixture of toner and carrier, it is necessary to sense the toner content of the developer correctly and control toner content by replenishing the toner consumed in the process of printing. Therefore, it is necessary to sense toner content of the developer correctly even for different kinds of developer to replenish correct amount of toner and to attain proper density of printing.
2. Description of the Related Art
FIG. 1(a) shows a block diagram of a developing unit of a related art of the present invention.
Controller 4a reads the sensor output voltage (called TDO hereafter) of a toner content sensor (called simply sensor hereafter) 3a which is provided in developer station 2a and compares TDO with the predesignated standard voltage to check to see if toner content is proper. If the toner content is less than predesignated amount, controller 4a controls toner hopper la to replenish developer station 2a with toner.
Sensor 3a, which senses toner content by measuring permeability of developer consisting of carrier and toner, gives permeability-output voltage (TDO) characteristics as shown in FIG. 1(b). When sensor 3a outputs 2.0 volts for desired toner content, 2.0 volts is designated as the standard voltage, that is, the toner content is high or low depending on whether sensor 3a outputs a voltage more or less than 2.0 volts, respectively. Meanwhile, developer is changed due to the degradation of carrier, that is, when carrier particle is deformed or becomes insufficiently charged. However, the developer may have different magnetic characteristics or offer different permeability for the same toner content. That is, sensor 3a may have TDO at point B or C, or sometimes D or C in FIG. 1(b) even for developer with the standard toner content. To overcome this problem, the conventional developing unit reads TDO of sensor 3a every time developer is changed and designated the TDO as the standard voltage (See, for example, Laid Opened Japanese Patent TOKUKAISHO 64-5299). However, if the standard value is designated at the above mentioned D or C, which is out of the linear portion of the characteristic curve, sensor 3a gives TDO voltage unproportional to the decrease in toner content and is apt to be influenced by noises, and therefore, preventing controller 4a from proper toner content control.